[dnanian]

What I'd do is CHECK YOUR BACKUP FIRST (start up from it). If you're happy with it, in general, then clean install Leopard (assumed) or Tiger to the internal. It'll prompt you to "copy from another Mac" during the install. Point that at the backup volume and it'll bring in your applications and data, leaving the fresh OS in place.

Alternatively, have you tried an 'archive and install'? It's very unusual to have a system get "more buggy over time" like this, and often it's best to diagnose what's wrong rather than toss the whole thing.